In the case of radio communication between two stations of which at least one is moveable, so-called Rayleigh fading will often occur. This is due to reflection of the radio signals against different objects, such that several signals which have travelled along different paths will mutually coact or counteract one another at different points. Consequently, the strengths of the signals received will vary in accordance with the mutual positions of the transmitter antenna and the receiver antenna, and may temporarily drop to zero or to a value in the vicinity of zero. Reflection of the signals also causes the direction of polarization of the transmitted radio wave to change in accordance with the form of the reflective object. The rate at which signal strength varies is proportional to the relative speed between the stations. On those occasions when the relative speed is zero, the connection may be broken because the input signal of the receiver in one station is excessively low. For example, this can occur in a mobile telephony system when a connection is established from a vehicle which remains stationary in a geographic location where the received signal strength is very low, or when the vehicle moves slowly in such a location. Such locations are called zero points.
It is known that poor reception due to the occurrence of zero points can be avoided with the aid of so-called space diversity. In this case, a station, for example a base station in a mobile telephony system, is equipped with two receiver antennas which are spaced appropriately apart. A so-called diversity effect is obtained in the fixed station with the aid of a suitable addition function of two received signals. This is based on the assumption that the probability of both antennas being located simultaneously in a zero point is small.
In time multiplex mobile telephony systems with digital modulation, the information from each subscriber is divided into information sections, called information frames. These frames are normally called speech frames in speech transmission. In the case of GSM-type systems (Groupe Speciale Mobile), the information in each such frame is divided into a specific number of signal sequences (bursts), which together with corresponding signal sequences from other subscribers form time slots in a plurality of mutually sequential TDMA-frames. Thus, each TDMA-frame includes information parts, i.e. signal sequences, from several subscribers. The number of signal sequences into which the information contained in each information frame is divided is determined by the number of so-called interleaving levels, the so-called interleaving depth in the channel code. In the GSM-system, the interleaving depth is equal to eight, i.e. the content of each information frame is divided into eight signal sequences.
It is known to apply frequency-jump transmission in GSM-type systems. In this case, different TDMA-frames are transmitted on different frequencies. This method is based on the fact that the positions of the zero points are frequency-dependent and that the probability of a geographic location being a zero point at more than one frequency is small when the frequency jump is sufficiently large. The method is suitable for GSM-type systems with deep interleaving.
In radio systems having small cell radii and therewith relatively short transmission distances, for example PCN-type systems (Personal Communication Network), a frequency jump does not provide any marked improvement of the reception at zero points. This is due to small time dispersion, i.e. there is a small time difference between signals that are received subsequent to having travelled through different paths to the receiver. Frequency jumps of reasonable magnitudes do not change the geographic positions of the zero points to any appreciable extent in such systems.